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Growing herb plants can be rewarding, providing you with fresh, flavorful herbs for cooking or fragrant herbs for sachets. Whether you keep your herb plants in an outdoor garden and/or grow potted herbs indoors, occasionally they may have yellow leaves, a possible sign of a cultural problem or an indicator of an insect problem or a plant disease. Identifying the reason for the yellow leaves is the first step in curing the problem and boosting your plants' health.

Temperature Shock

Sometimes herb plants develop yellow leaves because of a temporary environmental problem that resolves itself or that you can easily correct. For example, basil (Ocimum basilicum) grows as an annual plant in all parts of the United States, but temperatures might be cool and the air damp when it is planted outdoors in early spring. Those conditions can cause the leaves of basil and other herbs to turn yellow, a problem that is especially likely when a greenhouse-grown seedling is planted directly into a garden, shocking the plant with a sudden drop in temperature. Covering young herb plants with row covers for the first few weeks after they were planted can help keep the plants warm, as can hardening off tender seedlings by placing them in a warm, sheltered spot outdoors and moving them into a cooler spot for increasing periods of time each day. Do this for one to two weeks before planting them in the ground.

Water and Soil Problems

Yellow leaves on herb plants may also indicate a lack of water or nutrients, causing the plants to shed some of their leaves, which typically turn yellow before falling. For example, leaves at the base of a potted rosemary plant (Rosmarinus officinalis) might turn yellow because the plant is root bound, with a tight ball of roots that can't access soil water and nutrients efficiently. A plant in this condition should be repotted into a larger container, loosening the root ball and adding fresh potting soil. Rosemary is a perennial in U.S Department of Agriculture plant hardiness zones 8 through 10, and when it and other herbs grow in a garden, they may begin turning yellow during hot summer weather when they need extra water. Ensure plants get at least 1 inch of water weekly, including rain, and add a 2- to 3-inch-thick layer of organic mulch, such as straw or shredded bark, on the soil surface under each plant's canopy to conserve moisture. Keep the mulch back a bit from each plant's center to discourage fungal growth.

Fungal Disorders

If leaves on your outdoor-grown herb plants wilt and then begin turning yellow, it could signal a fungal disorder, such as one of several types of root or crown rot which can cause problems first on a single stem and gradually spread to other plant parts. The best way to prevent fungal problems in plants is by planting in an area with good air circulation and well-drained soil; if your garden's soil tends to stay soggy, you could also dig up an herb plant as soon as the problem appears on it, gently remove soil from its roots, add coarse sand to the area's soil and replant the herb. Fungal problems in potted herbs are prevented best by allowing the plants to drain well after watering their soil, never keeping them in water-filled saucers, and by keeping them well-spaced to ensure good air circulation.

Pest Problems

Some pests can infest herb plants and cause yellowing of leaves. For example, spider mites may cause leaves to yellow and curl as they suck plant juices. These microscopic pests construct visible, weblike coverings on young leaves and stems; spider mite problems often occur outdoors in hot, dry weather or indoors in dry, heated or air-conditioned air. Aphids also suck plant juices, especially from tender young growth, causing it to yellow, wilt and curl. Whitish-yellow insects visible to the naked eye, aphids are especially attracted to leaf undersides; they usually attack outdoors but can affect indoor plants, including potted herbs. Aphids and spider mites can be treated with insecticidal soap diluted at a rate of 6 tablespoons per 1 gallon of water. Spray the solution on an affected plant until it is dripping wet, and repeat the treatment every two weeks as needed. Rinse herbs well under running water to remove soap residue before using them as food.

About the Author

Joanne Marie began writing professionally in 1981. Her work has appeared in health, medical and scientific publications such as Endocrinology and Journal of Cell Biology. She has also published in hobbyist offerings such as The Hobstarand The Bagpiper. Marie is a certified master gardener and has a Ph.D. in anatomy from Temple University School of Medicine.